Write tests for slim format conversion helpers

[scorbajio]

This change introduces tests for util package account slim-format conversion helpers.

[codecov]

Codecov Report

Merging #3114 (3e4870e) into master (eeb74e4) will decrease coverage by 6.93%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

Flag	Coverage Δ
block	88.78% <ø> (?)
blockchain	92.27% <ø> (ø)
client	87.51% <ø> (?)
common	98.19% <ø> (ø)
ethash	∅ <ø> (?)
evm	71.87% <ø> (?)
rlp	∅ <ø> (∅)
statemanager	90.30% <ø> (?)
trie	90.32% <ø> (?)
tx	96.36% <ø> (?)
util	87.70% <ø> (+0.73%)	⬆️
vm	76.21% <ø> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

[holgerd77]

Updated this via UI

[holgerd77]

Something is really totally off with our Codecov settings!

How can codecov (project!!!) report -6.93% for THIS PR??? 🤔 🤔 🤔

[holgerd77]

Pretty simple and clean tests, nice! 👍

Will merge.